Assessing the fidelity of marine vertebrate microfossil δ18O signatures and their potential for palaeo-ecological and -climatic reconstructions

Abstract

Conodont biogenic apatite has become a preferred analytical target for oxygen isotope studies investigating ocean temperature and palaeoclimate changes in the Palaeozoic. Despite the growing application in geochemically-based palaeoenvironmental reconstructions, the paucity or absence of conodont fossils in certain facies necessitates greater flexibility in selection of robust oxygen-bearing compounds for analysis. Vertebrate microfossils (teeth, dermal denticles, spines) offer a potential substitute for conodonts from the middle Palaeozoic. Vertebrate bioapatite is particularly advantageous given a fossil record extending to the present with representatives across freshwater to fully marine environments, thus widening the scope of oxygen isotope studies on bioapatite. However, significant tissue heterogeneity within vertebrates and differential susceptibility of these tissues to diagenetic alteration have been raised as potential problems affecting the reliability of the oxygen isotope ratios as palaeoclimatic proxies. Well-preserved vertebrate microfossils and co-occurring conodont fossils from the Upper Devonian and Lower Carboniferous of the Lennard Shelf, Canning Basin, Western Australia, were analysed using bulk (gas isotope ratio mass spectrometry, GIRMS) and in-situ (secondary ion mass spectrometry, SIMS) methodologies, with the latter technique allowing investigation of specific tissues within vertebrate elements. The δ 18 O conodont results may be interpreted in terms of palaeolatitudinally and environmentally sensible palaeo-salinity and-temperature and provide a baseline standard for comparison against vertebrate microfossil δ 18 O values. Despite an absence of obvious diagenetic modification, GIRMS of vertebrate denticles yielded δ 18 O values depleted in 18 O by 2-4 ‰ relative to co-occurring conodonts. SIMS analysis of dentine tissues exhibited significant heterogeneity, while hypermineralised tissues in both scales and teeth produced δ 18 O values comparable with those of associated conodonts. The susceptibility of permeable phosphatic fossil tissues to microbial activity, fluid interaction and introduction of mineral precipitates post-formation is demonstrated in the dentine of vertebrate microfossils, which showed significant heterogeneity and consistent depletion in 18 O relative to conodonts. The hypermineralised tissues present in both teeth and scales appear resistant to many diagenetic processes and indicate potential for palaeoclimatic reconstructions and palaeoecological investigations.

Regional endothermy as a trigger for gigantism in some extinct macropredatory sharks

“…In fact, there are previous Rare Earth Elements analyses [ 40 , 63 – 65 ] as well as taphonomic evidence [ 38 , 39 ] for almost all the teeth included in this analysis supporting the lack of diagenetic processes (see [ 66 ] for a detailed review on the applicability of REE analysis as a tool for detecting diagenetic processes). Moreover, Žigaitė and Whitehouse [ 67 ] have recently studied the isotopic composition of different tooth tissues in extant sharks demonstrating that enameloid shows less δ 18 O variation than dentine and recommended it as a target biomineral and as a preferential biogeochemical reference for environmental and paleoenvironmental studies (see also [ 68 ]). Secondly, δ 18 O differences between teeth of co-occurring individuals can also reflect differences in water column temperature due to distinct water depth preferences [ 40 , 69 , 70 ].…”

High‐Mg# Olivine, Clinopyroxene and Orthopyroxene Reference Materials for In Situ Oxygen Isotope Determination

“…The analytical method used in IGGCAS is similar to the descriptions in Li et al () and Tang et al (), except that a ~ 133 transfer magnification was employed to increase secondary ion transmission. The measurement conducted in GIGCAS and UWA followed the method described by Yang et al () and Roelofs et al (). The instrument configurations in the three SIMS laboratories were similar.…”

Ultrastructure, composition, and 87Sr/86Sr dating of shark teeth from lower Miocene sediments of southwestern Peru

“…In particular, Sr isotope measurements of the enameloid layer give Sr ages that are more reliable than observed for dentine (Becker et al, 2008;Tütken et al, 2020). This is due to the resistance of enameloid to diagenetic processes and weathering (Roelofs et al, 2017), similar to the enamel of mammalian teeth (Ayliffe et al, 1994;Kohn et al, 1999;Zazzo et al, 2004;Tütken et al, 2008). Dentine is therefore more prone to recrystallize during diagenesis than enameloid and other hypermineralized tissues that are J o u r n a l P r e -p r o o f more resistant to diagenetic alteration (Roelofs et al, 2017).…”

“…This is due to the resistance of enameloid to diagenetic processes and weathering (Roelofs et al, 2017), similar to the enamel of mammalian teeth (Ayliffe et al, 1994;Kohn et al, 1999;Zazzo et al, 2004;Tütken et al, 2008). Dentine is therefore more prone to recrystallize during diagenesis than enameloid and other hypermineralized tissues that are J o u r n a l P r e -p r o o f more resistant to diagenetic alteration (Roelofs et al, 2017). However, it must be pointed out that the application of SIS on shark teeth from the Cenozoic successions comports some limitations as concerns the Paleogene, due to the oscillation and flattening of the calibration curve in correspondence of this system/period (McArthur et al, 2020).…”